1. Field of the Invention
This invention relates generally to digital x-ray imaging and, more particularly, relates to methods and apparatuses for reducing scatter in two-dimensional x-ray imaging and two-dimensional dual-energy x-ray imaging.
2. The Prior Art
Recent advances in the field of semiconductor fabrication have resulted in the ability to fabricate large-format two-dimensional integrated detector arrays for x-ray detection. These arrays have on the order of one million detector cells and provide instant acquisition of two-dimensional x-ray images with exceedingly high quality.
Scatter, which results from those x-rays that strike objects and deflect in random directions, has been a difficult and on-going problem in x-ray imaging using two-dimensional detectors. For example, in projection chest radiography, scatter typically accounts for between approximately 30% and 50% of the total amount of x-rays detected.
With single-point detectors or linear detector arrays, because of the inherent geometric configuration of the detector, scatter can be controlled so that its effects are negligible. However, two-dimensional detectors are exposed to wide-angle random scatter. Randomly scattered x-rays are superimposed on the primary x-rays (the x-rays coming directly from the x-ray source) and recorded by two-dimensional detectors undifferentiated, degrading the true image. Scatter tends to reduce image contrast, produce blurring, and reduce signal-to-noise ratio. Furthermore, if scatter is not substantially reduced, almost all quantitative digital x-ray imaging using two-dimensional detector arrays becomes meaningless. For example, currently, dual-energy x-ray imaging, which is a method for determining two material composition images of a subject, can only be conducted using point or linear detector array scanning to gain a two-dimensional image. So, unless scatter is substantially removed or eliminated, two-dimensional detector arrays cannot be used in dual-energy x-ray imaging.
According to current theory and empirically derived data, a large number of complex material systems can be decomposed into only two basis material compositions in terms of x-ray absorption. In the case of the human body, these two materials are bone tissue and soft tissue, abbreviated b and s, respectively. The prior art of data decomposition methods is summarized in Keh-Shih Chung & H. K. Huang, Comparison of Four Dual-energy Image Decomposition Methods, 4 Physics in Medicine and Biology 455 (1988), and in the book Heinz W. Wahner & Ignac Fogelman, The Evaluation of Osteoporosis: Dual-energy X-ray Absorptiometery in Clinical Practice 14-33 (1994). All the data decomposition methods of the prior art have a common approach. They all use two single-energy values to replace the broad spectrum x-ray energy: the average energy E.sub.H for high-energy x-rays and the average energy E.sub.L for low-energy x-rays. Thus, the dual-energy equations are greatly simplified into a pair of linear algebraic equations that can be readily solved for b and s: EQU I.sub.H =I.sub.H0 .times.exp(-(.mu..sub.b (E.sub.H).times.b+.mu..sub.s (E.sub.H).times.s)) (1a) EQU I.sub.L =I.sub.L0 .times.exp(-(.mu..sub.b (E.sub.L).times.b+.mu..sub.s (E.sub.L).times.s)) (1b)
where I.sub.H0 and I.sub.L0 are the incident x-ray beam intensities at energies E.sub.H and E.sub.L, respectively, I.sub.H and I.sub.L are the measured signals read from high-energy and low-energy detectors, respectively, .mu..sub.b (E.sub.H) and .mu..sub.b (E.sub.L) are the mass absorption coefficients of bone tissue of high-energy and low energy x-rays, respectively, and .mu..sub.s (E.sub.H) and .mu..sub.s (E.sub.L) are the mass absorption coefficients of soft tissue of high-energy and low energy x-rays, respectively. Taking the natural logarithm of equation pair (1a,1b) yields EQU L.sub.H =ln(I.sub.H /I.sub.H0)=-(.mu..sub.b (E.sub.H).times.b+.mu..sub.s (E.sub.H).times.s) (2a) EQU L.sub.L =ln(I.sub.L /I.sub.L0)=-(.mu..sub.b (E.sub.L).times.b+.mu.s(E.sub.L).times.s) (2b)
Thus, b and s can be analytically determined as simple functions of experimental data L.sub.H and L.sub.L.
In most cases, because of so-called "beam hardening effects", the results directly calculated from the linearized equation pair (2a,2b) deviate too much from reality. Therefore, the results are subjected to numerous correction methods. These different correction methods account for the various data decomposition methods of the prior art. In some correction methods, the corrections extend to the second order. The data decomposition methods of prior art can be used in certain specific cases. For example, when measuring certain fixed points in the human body, the x-ray absorption varies only within a narrow range. However, for a human body with an average thickness of between 20 cm and 30 cm, the absorption of x-rays can vary greatly. The intensity can be being reduced by a small fraction of its incident intensity to as high as several hundred times. And the x-ray energy spectra as well as the average x-ray energy values change dramatically from one position to another. Thus, some inconsistencies arise from the linearization approach.
Currently, there are three basic methods for reducing scatter in two-dimensional x-ray imaging. The first method uses an anti-scatter grid to slightly relieve scatter effects on images. An anti-scatter grid consists of large number of fine wires placed in front of the detector. Because the grid has a certain amount of collimating ability, the randomly scattered x-rays can be somewhat reduced. However, the grid also tends to block the primary x-rays, causing distortion of the primary image. Thus, the grid must be thin, limiting its ability to reduce scatter. Recent research results show that up to about 50% of the scatter radiation can be reduced through use of an anti-scattering grid.
The second method to reduce scatter is to increase the air gap between the subject and the detector. The scatter is attenuated, but the image is blurred due to the geometric distance the x-rays have to travel.
The third method to reduce scatter is to calculate theoretical estimates of the amount of scatter and subtract these estimates from the detected image. Theoretical calculation methods, including Monte Carlo simulation methods and analytical deconvolution methods, can only give very crude predictions, and are not generally considered effective.
Thus, there continues to be a need to accurately remove scatter effects from images detected by large-format two-dimensional x-ray detector arrays and to produce scatter-free dual-energy x-ray images from these arrays.